Coaxial cables have been used for years in cabling systems providing different television channels to homes. Today, coaxial cables are also used to provide a growing number of residences with packet-based high-speed Internet access. Cabling systems may today apply packet-based technologies that further enable home owners to make telephone calls and to receive programming technologies over their coaxial cable infrastructure, thereby creating a home network.
Cable equipment, and in particular coaxial cables, have been extensively used in home environments, as they have been designed to shield signals carried on the cable from outside interference. Television sets are also designed to only accept signals from the point of connection to the cable or antenna. However, connectors used in any of these systems may still allow for interference to enter the systems, especially in analogue television systems, which may result in a double television image (“ghosting”) or signal attenuation.
Historically, very long cabling systems comprising coaxial cables connected various antennas to the television sets of subscribers. To overcome the weakening of the signals across the coaxial cables, amplifiers were used at regular intervals across the cabling systems to amplify or boost the signal strength. This ensured a strong enough analogue signal for television viewing.
Passive devices such as splitters, diplexers or combiners, are devices used in coaxial cabling systems that allow signals (e.g., direct broadcast satellite TV signals from a dish to the receiver) to piggyback on one regular coaxial cable, along with lower-frequency signals from an outdoor terrestrial TV antenna for local channels. This is useful in homes which are pre-wired, as it eliminates the need for unsightly extra cables. A diplexer typically joins or separates two signals, to be used by different components such as a receiver/decoder (IRD) of the direct broadcast satellite (DBS) set-top box and a direct feed to the TV for example.
Other passive devices, typically called splitters (e.g. two way, three way, four way, six way, or eight way splitters) or directional couplers, allow signals to be transmitted from a main source to various devices, such as television sets or set-top boxes, within a home entertainment system. The typical communication across these splitters or directional couplers is input-to-multiple outputs, or multiple outputs to a single input.
The output-to-input or input-to-output configuration of splitters ensures a high output port-to-output port isolation, e.g. to prevent a double image. Connectors are therefore carefully designed and tuned to prevent intermodulation and keep reflected power to a minimum for each input transmitter and frequency. In particular, connectors used in analogue cabling systems for in-home usage typically make use of impedance matching transformers. Passive devices therefore usually have the same impedance as the cables to which they are connected and may further have a similar cutoff frequency. Similarly, the impedance of cables may typically be the same as the load, e.g. the television sets. As is well known, whenever the source of power operates into a load, the greatest power is delivered to the load when the impedance of the load is equal to the resistance of the source.
This configuration has the implication that no signal can be sent from one output to another output of a splitter or directional coupler, as the dB loss, ensuring port-to-port isolation, is too high.